Microbial infections in animals, such as mastitis in dairy cows, are typically treated or prevented during the lactation period by, intramammary (IMM) infusion, intramuscular (IM) or subcutaneous (SC) injection.
Typically, antibiotics such as penicillins or prodrugs of benzylpenicillin (BP) such as penethamate (PNT) have been used as actives of choice to treat bovine mastitis. PNT is the diethylaminoethyl ester of benzylpenicillin. In formulations intended for veterinary use PNT is incorporated as the hydroiodide (HI). PNT HI is used in intramammary products during lactation (UBRO YELLOW™, Boehringer Ingelheim) and during the dry-off period (UBRO RED™, Boehringer Ingelheim), as well as an injectable suspension during lactation period (Mamyzin™, Boehringer Ingelheim and Penethaject™, Bayer Animal Health) for treatment of mastitis in cows.
PNT is a prodrug from which benzylpenicillin and diethylaminoethanol are released by hydrolysis. Antimicrobial activity of the compound is exclusively related to benzylpenicillin. The maximum residue limit (MRL) for milk in bovine in e.g. Europe and New Zealand is 4 μg/kg (EMEA, BP).
As a prodrug of benzylpenicillin, penethamate hydroiodide is effective in treating mastitis as a result of its particular pharmacokinetics. After intramuscular administration, penethamate hydroiodide is absorbed from the site of injection and on entering the blood partially dissociates by hydrolysis into benzylpenicillin and diethylaminoethanol. At blood pH (7.2), an equilibrium is established wherein 91.8% of the active drug is present in its hydrolysed form (benzylpenicillin) with the remainder being penethamate. The equilibrium is maintained by re-association of benzylpenicillin and diethylaminoethanol. Peak serum levels (measured as dissociated penicillin G) are rapidly reached, 3.76 hours after injection (Friton 2003).
The un-disassociated form of penethamate easily passes over the blood-milk barrier due to the pH gradient present between milk (pH 6.6-6.8) and plasma (pH 7.2) and its weakly basic state (pKa=8.4). The lipophilic nature of penethamate further facilitates its passage across the lipo-proteineic blood-milk barrier. Penethamate starts to dissociate as it passes over the blood-milk barrier and this continues during diffusion of the drug throughout the udder, releasing benzylpenicillin. The benzylpenicillin is rapidly ionised in the udder (pKa=2.8), as a result of the lower pH of milk, trapping the active within the udder in increasing concentrations.
Current PNT products available on the market for intramuscular (IM) or sub-cutaneous (SC) injection, are in the form of a powder for reconstitution into solution or suspension with sterile water at the time of use (e.g. Mamyzin™ Boehringer Ingelheim or Penethaject™ Bayer Animal Health). Typical dosages of these products include three daily doses of 5 g of PNT, or one dose of 10 g followed by a 5 g dose the next day.
The use of an aqueous vehicle allows for the PNT to dissolve quickly at the injection site and thus be rapidly absorbed. Especially desirable properties for treating mastitis during the lactation period include rapid absorption of the active, effective therapeutic action, and a short withhold period. The ability of penethamate to cross the blood/milk barrier and concentrate in the udder, provides effective therapeutic action and a sufficiently short withhold period.
Despite their effectiveness as therapeutic actives, a disadvantage of currently available compositions as injections containing penicillin or PNT is their limited shelf-life (often only 2-3 days) once reconstituted into an aqueous solution from a powder. This is inconvenient to the user, who must reconstitute the powder into a liquid form, such as by drawing out a sterile aqueous vehicle from one vial, dispense the liquid into a second vial containing the powder and mix until a solution or homogeneous suspension is formed. Once the product is reconstituted it must be used within the limited period of stability, or it must be discarded.
Currently, there is no ready to use IM/SC injectable formulation of PNT to treat bovine mastitis or other diseases available on the market. A significant problem in developing a ready to use injectable formulation is the lack of storage stability of PNT in aqueous vehicles. The use of non-aqueous vehicles has been avoided for PNT formulations, since oil vehicles in particular typically provide a much slower release of the active after IM/SC injection. A slow release of the active would extend the withhold period for lactating cows, which is especially undesirable and would likely prevent commercialisation of the product.
The published patent document U.S. Pat. No. 4,446,144 mentions that PNT can be used in suspensions or solutions in suitable vehicle which can be made of an aqueous or oily base. Non-aqueous vehicles are indicated as providing better stability. The formulations described in U.S. Pat. No. 4,446,144 are for parenteral use, e.g. injections given as an aqueous solution or suspension. No mention is made of the desirability of obtaining fast-release of the active or bioequivalence with aqueous formulations.
There has been limited research in formulating a PNT composition for a IM/SC injection using a non-aqueous vehicle. Edwards, S. J. (1964), The Veterinary Record, Vol. 78, No. 17, 583-5, documents a study of penicillin levels in milk following intramuscular injection.
The prior art products requiring reconstitution of PNT into an aqueous vehicle provide a compromise, with the advantage of the rapid absorption of the active when administered, with the disadvantage of the limited storage stability of the reconstituted composition.
There has been a long felt need for a PNT composition that is ready to use, has good storage stability, and rapid release of the active.
It is an object of the present invention to address the foregoing problems or at least to provide the public with a useful choice.
All references, including any patents or patent applications cited in this specification are hereby incorporated by reference. No admission is made that any reference constitutes prior art. The discussion of the references states what their authors assert, and the applicants reserve the right to challenge the accuracy and pertinency of the cited documents. It will be clearly understood that, although a number of prior art publications are referred to herein, this reference does not constitute an admission that any of these documents form part of the common general knowledge in the art, in New Zealand or in any other country.
Throughout this specification, the word “comprise”, or variations thereof such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated element, integer or step, or group of elements integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.
Further aspects and advantages of the present invention will become apparent from the ensuing description which is given by way of example only.